The invention relates to a method and device for scrambling-unscrambling digital image data.
With the recent appearance of compression and decompression techniques using, in particular discrete cosine transforms, DCT, new applications and commercial services can develop for using these techniques in the digital transmission field (digital television with bit rate reduction or transport of fixed images over service integration digital networks SIDN) or even in the field of optimizing data storage media (documentary data bases). By way of example of recent developments based on the DCT techniques, different actions may be mentioned for standardizing digital image transmission systems:
ADCT-ISO coding of photographic images for a multimedia telematic service over SIDN, PA0 reduction codecs (CMTT) for the transmission of digital data at bit rates of 32 to 45 Mbits/s.
Although the technical obstacle for maintaining the quality of the image after encoding has at the present time been overcome, the commercial use of associated services may be hindered by problems of protection of the rights of the authority providing the transmission of images, such authority desiring either to ensure the confidentiality of such transmissions, or receive fees for access to the reception of the final image.
Such problems of protecting the transmission or reserving the right of access to the use of images may be solved on condition that digital image scrambling-unscrambling systems are available used preferably under the direct control of the owner operators of the image transmission, i.e. before and after transmission and broadcasting, or under the control of service providers, prime contractors in the digital transmission chain.
At the present time, several image scrambling techniques are applied in the analog television signal transmission field.
These techniques, used at transmission, would require, for application to digital television signals, D-A conversion before scrambling then A-D conversion before transmission.
Similarly, at reception, it is necessary to provide D-A conversion then A-D conversion for unscrambling the image. Consequently, without prejudging the compatibility of the digital image compression-decompression techniques used for digital transmission, the quality of the image will thus be degraded by the multiple analog and digital conversion steps.
The above techniques are therefore difficult to apply to digital transmission.
The above techniques, particularly the transposition in the digital field of television scrambling-unscrambling techniques, namely horizontal random translation of the active video lines and D2 MAC / Packets (rotation of the contents of the active video lines with random cut-off point) is incompatible with maintaining the video quality obtained, because of the DCT type compression processing.
In fact, the purpose of such techniques is to destroy the correlation between the successive lines of the image, whereas exploitation of the correlation between adjacent pixels is the characteristic of the discrete cosine transform DCT.
More recently, a block line permutation process has been proposed by the French patent application 88 00361 filed in the name of Telediffusion De France, the assignee of the present applicant, on 14th Jan. 1988 and corresponding to European Patent No. 325509. Although the processing described in said patent relates to applications relative to digital television images, this type of scrambling-unscrambling requires the use of a digital image memory.
Furthermore, restrictions in use of this type of process must be anticipated for satisfying the particular DCT downstream processing conditions, such restrictions in particular facilitating penetration of the scrambling device by unauthorized third parties.